Twins who had been smoking for less time, or not at all, had significantly younger-looking faces. The smokers had upper eyelids that drooped, lower lids that sagged, increased wrinkling around the mouth and sagging facial skin, or jowls. The differences were more prominent in the twins' middle and lower faces than in their upper faces, the study found.

Even for pairs where both twins were smokers, the study found that the twin who had been smoking longer had noticeable signs of aging that the identical sibling did not have.

"It is noteworthy that even among sets of twins where both are smokers, a difference in five years or more of smoking duration can cause visibly identifiable changes in facial aging," Dr. Guyuron wrote in the paper.